Beat butter or margarine in a large mixing bowl with an electric mixer on medium to high speed for 30 seconds. Add sugar, baking powder, baking soda, and salt; beat till combined, scraping sides of bowl occasionally. Beat in the egg, mashed bananas, sour cream, and vanilla till combined. Beat in as much of the flour as you can with the mixer. Stir in remaining flour and almonds with a wooden spoon.

Pour the batter into a greased 15x10x1-inch baking pan, spreading evenly. Bake in a 350 degrees oven about 25 minutes or till a wooden toothpick inserted near the center comes out clean. Cool in pan on a wire rack.

Spread with Butter-Rum Frosting. Cut into bars. Just before serving, sprinkle with freshly grated nutmeg, if desired. Makes 48 bars.